Oecologia DOI 10.1007/s00442-013-2848-8
Physiological ecology - Original research
Overlap in nitrogen sources and redistribution of nitrogen between trees and grasses in a semi‑arid savanna K. V. R. Priyadarshini · Herbert H. T. Prins · Steven de Bie · Ignas M. A. Heitkönig · Stephan Woodborne · Gerrit Gort · Kevin Kirkman · Brian Fry · Hans de Kroon
Received: 14 February 2013 / Accepted: 19 November 2013 © Springer-Verlag Berlin Heidelberg 2013
Abstract A key question in savanna ecology is how trees and grasses coexist under N limitation. We used N stable isotopes and N content to study N source partitioning across seasons from trees and associated grasses in a semiarid savanna. We also used 15N tracer additions to investigate possible redistribution of N by trees to grasses. Foliar stable N isotope ratio (δ15N) values were consistent with trees and grasses using mycorrhiza-supplied N in all seasons except in the wet season when they switched to microbially fixed N. The dependence of trees and grasses on mineralized soil N seemed highly unlikely based on seasonal variation in mineralization rates in the Kruger Park region. Remarkably, foliar δ15N values were similar for all three tree species differing in the potential for N fixation through
nodulation. The tracer experiment showed that N was redistributed by trees to understory grasses in all seasons. Our results suggest that the redistribution of N from trees to grasses and uptake of N was independent of water redistribution. Although there is overlap of N sources between trees and grasses, dependence on biological sources of N coupled with redistribution of subsoil N by trees may contribute to the coexistence of trees and grasses in semi-arid savannas.
Communicated by Allan Green.
Introduction
Electronic supplementary material The online version of this article (doi:10.1007/s00442-013-2848-8) contains supplementary material, which is available to authorized users.
Savanna soils are weathered, leached and deficient in N (Cole 1986; Scholes et al. 2003). The N content of savanna
K. V. R. Priyadarshini (*) · H. H. T. Prins · S. de Bie · I. M. A. Heitkönig Resource Ecology Group, Wageningen University, Wageningen, The Netherlands e-mail: [email protected]
G. Gort Biometris, Wageningen University, Wageningen, The Netherlands e-mail: [email protected]
H. H. T. Prins e-mail: [email protected] S. de Bie e-mail: [email protected] I. M. A. Heitkönig e-mail: [email protected] S. Woodborne iThemba Laboratories, WITS, Private Bag 11, Johannesburg 2050, South Africa e-mail: [email protected]
Keywords Tree–grass interactions · Nitrogen source · Nitrogen-15 stable isotope · Nitrogen-15 labelling · Andover Game Reserve · Nitrogen redistribution
K. Kirkman University of KwaZulu-Natal, Scottsville, Pietermaritzburg 3209, South Africa e-mail: [email protected] B. Fry Australian Rivers Institute, Griffith University, Brisbane, QLD 4111, Australia e-mail: [email protected] H. de Kroon Department of Experimental Plant Ecology, Institute for Water and Wetland Research, Radboud University, Nijmegen, The Netherlands e-mail: [email protected]
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Oecologia
soils across Africa is very low, typically 4–6 ‰ and close to or above soil N isotopic values indicate the use of mineralized N (Nadelhoffer and Fry 1994; Robinson 2001), while intermediate values ranging from +2 to +5 ‰ indicate the use of mycorrhizal N (Hogberg and Alexander 1995; Craine et al. 2009a) or a mixed use of N from fixation and soil sources. The foliar δ15N values were calculated with atmospheric N isotope ratio as the standard with the formula: δ15N = {[(15N/14NSAMPLE)/(15N/14NSTANDARD)] 1} × 1,000. The 15N/14N STANDARD represents the stable isotope ratio of the international standard for N which is atmospheric N and 15N/14NSAMPLE represents the stable isotope ratio of the sample. Labelling with 15N as a tracer A tracer solution was prepared comprising 5 g 15NH4Cl with a +99 ‰ (100 % 15N) isotope ratio of 15N and diluted with 1 l 2H2O (both tracers sourced from Icon Services, NJ). Results of 2H2O labelling are described elsewhere (Priyadarshini et al., in review). The deeper rooting zone
The youngest and fully expanded leaves of terminal ends of the upper canopy branches were sampled following Lehmann et al. (2001). Samples were collected from all cardinal directions of the upper tree canopy and composited into a single sample. Concurrently, fresh and new grass blades from 1-m radius around the base of the tree were collected from all cardinal directions. These were also composited into a single sample to represent the undercanopy grass for that tree. Plant material was sampled before applying the stable isotope (pre-spike) and then 1 week after the application of the isotope (spike) during all seasons. Therefore, pre-spike and spike samples are of the same tree before and after labelling, respectively. Material from control trees (control) was sampled similarly and concurrently as the experimental trees. Sampling was done during the wet season of 2010, the wet–dry transition season, the dry season, the dry–wet transition season, and the wet season of January 2011. Plant samples were oven dried to constant weight at less than 60 °C to prevent heat loss of N, finely ground, and stored until laboratory analysis. Sampling of soil Soil samples were collected at the beginning of the experiment from 0.25-m depth and 2.5-m depth in the four cardinal directions from the base of experimental and control trees. Soil was composited and a subsample was pulverized (Mintek Laboratories, South Africa). The pulverized soil was first treated with dilute HCl to remove the soil carbonates and thoroughly rinsed with de-ionized water and oven dried for further analysis in the mass spectrometer.
13
Oecologia
Determination of stable isotope ratios and %N Sample aliquots of approximately 1 mg of plant material or 40 mg of soil were weighed into clean tin capsules (Elemental Microanalysis, Okehamptom, UK) at the Council for Scientific and Industrial Research laboratory in Pretoria, South Africa using a micro-balance (Mettler-Toledo, OH). Samples were analysed using a Flash EA 1112 system coupled with a Delta V plus mass spectrometer using a Conflo IV interface (Thermo Electron, Bremen, Germany). Precision is
Physiological ecology - Original research
Overlap in nitrogen sources and redistribution of nitrogen between trees and grasses in a semi‑arid savanna K. V. R. Priyadarshini · Herbert H. T. Prins · Steven de Bie · Ignas M. A. Heitkönig · Stephan Woodborne · Gerrit Gort · Kevin Kirkman · Brian Fry · Hans de Kroon
Received: 14 February 2013 / Accepted: 19 November 2013 © Springer-Verlag Berlin Heidelberg 2013
Abstract A key question in savanna ecology is how trees and grasses coexist under N limitation. We used N stable isotopes and N content to study N source partitioning across seasons from trees and associated grasses in a semiarid savanna. We also used 15N tracer additions to investigate possible redistribution of N by trees to grasses. Foliar stable N isotope ratio (δ15N) values were consistent with trees and grasses using mycorrhiza-supplied N in all seasons except in the wet season when they switched to microbially fixed N. The dependence of trees and grasses on mineralized soil N seemed highly unlikely based on seasonal variation in mineralization rates in the Kruger Park region. Remarkably, foliar δ15N values were similar for all three tree species differing in the potential for N fixation through
nodulation. The tracer experiment showed that N was redistributed by trees to understory grasses in all seasons. Our results suggest that the redistribution of N from trees to grasses and uptake of N was independent of water redistribution. Although there is overlap of N sources between trees and grasses, dependence on biological sources of N coupled with redistribution of subsoil N by trees may contribute to the coexistence of trees and grasses in semi-arid savannas.
Communicated by Allan Green.
Introduction
Electronic supplementary material The online version of this article (doi:10.1007/s00442-013-2848-8) contains supplementary material, which is available to authorized users.
Savanna soils are weathered, leached and deficient in N (Cole 1986; Scholes et al. 2003). The N content of savanna
K. V. R. Priyadarshini (*) · H. H. T. Prins · S. de Bie · I. M. A. Heitkönig Resource Ecology Group, Wageningen University, Wageningen, The Netherlands e-mail: [email protected]
G. Gort Biometris, Wageningen University, Wageningen, The Netherlands e-mail: [email protected]
H. H. T. Prins e-mail: [email protected] S. de Bie e-mail: [email protected] I. M. A. Heitkönig e-mail: [email protected] S. Woodborne iThemba Laboratories, WITS, Private Bag 11, Johannesburg 2050, South Africa e-mail: [email protected]
Keywords Tree–grass interactions · Nitrogen source · Nitrogen-15 stable isotope · Nitrogen-15 labelling · Andover Game Reserve · Nitrogen redistribution
K. Kirkman University of KwaZulu-Natal, Scottsville, Pietermaritzburg 3209, South Africa e-mail: [email protected] B. Fry Australian Rivers Institute, Griffith University, Brisbane, QLD 4111, Australia e-mail: [email protected] H. de Kroon Department of Experimental Plant Ecology, Institute for Water and Wetland Research, Radboud University, Nijmegen, The Netherlands e-mail: [email protected]
13
Oecologia
soils across Africa is very low, typically 4–6 ‰ and close to or above soil N isotopic values indicate the use of mineralized N (Nadelhoffer and Fry 1994; Robinson 2001), while intermediate values ranging from +2 to +5 ‰ indicate the use of mycorrhizal N (Hogberg and Alexander 1995; Craine et al. 2009a) or a mixed use of N from fixation and soil sources. The foliar δ15N values were calculated with atmospheric N isotope ratio as the standard with the formula: δ15N = {[(15N/14NSAMPLE)/(15N/14NSTANDARD)] 1} × 1,000. The 15N/14N STANDARD represents the stable isotope ratio of the international standard for N which is atmospheric N and 15N/14NSAMPLE represents the stable isotope ratio of the sample. Labelling with 15N as a tracer A tracer solution was prepared comprising 5 g 15NH4Cl with a +99 ‰ (100 % 15N) isotope ratio of 15N and diluted with 1 l 2H2O (both tracers sourced from Icon Services, NJ). Results of 2H2O labelling are described elsewhere (Priyadarshini et al., in review). The deeper rooting zone
The youngest and fully expanded leaves of terminal ends of the upper canopy branches were sampled following Lehmann et al. (2001). Samples were collected from all cardinal directions of the upper tree canopy and composited into a single sample. Concurrently, fresh and new grass blades from 1-m radius around the base of the tree were collected from all cardinal directions. These were also composited into a single sample to represent the undercanopy grass for that tree. Plant material was sampled before applying the stable isotope (pre-spike) and then 1 week after the application of the isotope (spike) during all seasons. Therefore, pre-spike and spike samples are of the same tree before and after labelling, respectively. Material from control trees (control) was sampled similarly and concurrently as the experimental trees. Sampling was done during the wet season of 2010, the wet–dry transition season, the dry season, the dry–wet transition season, and the wet season of January 2011. Plant samples were oven dried to constant weight at less than 60 °C to prevent heat loss of N, finely ground, and stored until laboratory analysis. Sampling of soil Soil samples were collected at the beginning of the experiment from 0.25-m depth and 2.5-m depth in the four cardinal directions from the base of experimental and control trees. Soil was composited and a subsample was pulverized (Mintek Laboratories, South Africa). The pulverized soil was first treated with dilute HCl to remove the soil carbonates and thoroughly rinsed with de-ionized water and oven dried for further analysis in the mass spectrometer.
13
Oecologia
Determination of stable isotope ratios and %N Sample aliquots of approximately 1 mg of plant material or 40 mg of soil were weighed into clean tin capsules (Elemental Microanalysis, Okehamptom, UK) at the Council for Scientific and Industrial Research laboratory in Pretoria, South Africa using a micro-balance (Mettler-Toledo, OH). Samples were analysed using a Flash EA 1112 system coupled with a Delta V plus mass spectrometer using a Conflo IV interface (Thermo Electron, Bremen, Germany). Precision is
